Irradiated, cross-linked polyethylene copolymer



United States Patent 3,097,150 IRRADIATED, CROSS-LINKED POLYETHYLENE COPOLYMER William C. Rainer, Edward M. Redding, and Joseph J.

Hitov, Baltimore, Md., and Arthur W. Sloan and William D. Stewart, Alexandria, Va., assignors, by mesne assignments, to W. R. Grace & Co., Cambridge, Mass, a corporation of Connecticut No Drawing. Filed June 24, 1955, Ser. No. 517,943 5 Claims. (Cl. 204154) The present invention relates to condensation products of polyethylene with other polymers and ethylenically unsaturated monomers.

Polyethylene is widely used today in making containers, toys, etc. Despite its many advantages, it suffers from the disadvantage that it is too flexible and does not have suflicient strength for some uses. Additionally, it is a relatively expensive material. Certain other polymers, such as Butyl rubber, while being less expensive than polyethylene, likewise do not have suflicient strength for some uses and also are insufficiently rigid. Polyisobutylene (Vistanex) also does not have sufficient strength and is insufficiently rigid.

It has previously been proposed to cross-link polyethylene by high voltage irradiation to render the polyethylene more rigid. However, for some purposes, this procedure still left something to be desired.

Attempts have also been made to cross-like other polyrners by irradiation, but such attempts have not been uniformly successful. Thus, materials, such as plasticized cellulose acetate, Butyl rubber (isobutylene-isoprene or butadiene copolymer) deteriorate when irradiated with a 2 m.e.v. electron beam in a dosage of 20x10 REP at room temperature. Similarly, polyisobutylene deteriorates under the same conditions.

It is thus impossible to predict whether irradiation will increase, decrease or have any elfect at all on the mixture of two or more polymeric materials.

It is a primary object of the present invention to prepare condensation products of polyethylene with other non-polar or slightly polar polymers and ethylenically unsaturated monomers having inoreased toughness and rigidity.

Another object of the invention is to reduce discoloring in graft polymerization.

Still further objects and the entire scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

It has now been found that these objects can be attained and new cross-linked polymers of increased strength and rigidity can be obtained if polyethylene is irradiated in the presence of other non-polar or only slightly polar polymers or ethylenically unsaturated monomers. Preferably, the monomers or polymers are hydrocarbons. The invention is of particular advantage with polymers of isobutylene. It is, in fact, surprising that polyisobutylene will react with polyethylene during irradiation to give a cross-linked product of increased strength and rigidity, as polyisobutylene itself and Butyl rubber are deteriorated by the irradiation treatment in the absence of the polyethylene.

The products of the present invention can be molded in conventional manner, e.g., compression, extrusion, transfer and injection molding procedures can be employed.

The polyethylene can be interpolymerized with one or more other substantially non-polar polymers. Thus, the polyethylene can be replaced with up to e.g., 50%, of another non-polar polymer. When polyethylene is the predominant polymer, it is, preferably, used in an amount of 70 to 95% of the total polymers. When polyethylene is used as the minor constituent, it is present in an amount of about 5 to 30% of the total polymers.

As the modifying polymer, either as a minor constituent or as a major constituent, there can be employed solid polymers of ethylenically unsaturated hydrocarbons, such as polyisobutylene, isobutylene copolymerized with 1 to 10% of a diolefin, such as butadiene or isoprene, e.g., a 97% to 3% copolymer of isobutylene with isoprene, and other materials coming within the term Butyl rubber. Patent No. 2,356,128 gives typical examples of such polymers. There can also be used polybutadiene, polyisoprene, polychloroprene, natural rubber, polystyrene, copolymers of styrene with isobutylene containing a preponderant amount of styrene, such as Stybutene, polymers of the isomeric vinyl toluenes, e.g., ortho vinyl toluene, para vinyl toluene and meta vinyl toluene, polybutene, polymeric u-methyl styrene, terpene polymers, such myrcene polymers, rubbery butadiene-styrene copolymers (GRS) etc. In place of these solid polymers, there can be used liquid polymers, such as natural rubber degraded to a molecular weight of about 5000 to 8000. Other liquid polymers which can be used are liquid GR-S (5000 to 10,000 molecular weight), dicyclopentadiene, liquid butadiene-acrylonitrile low in acrylonitrile (10% to 20% acrylonitrile). There also can be used a copolymer of butadiene or other diolefins with a minor amount of methyl vinyl ketone.

Instead of mixing polyethylene with any of the above materials, it is also possible to employ grafted polymers, wherein ethylenically unsaturated monomers are grafted to polyethylene. Suitable monomers for grafting purposes include isoprene, cyclopentadiene, styrene, ormethyl styrene, alkyl substituted styrenes, such as o-vinyl toluene, p-vinyl toluene, m-vinyl toluene, and p-ethyl styrene, dialkenyl benzenes, e.g., para-divinyl benzene, ring halogenated styrenes, such as o-chlorostyrene, pchlorostyrene, 2,4-dichlorostyrene and 2,6-dichlorostyrene, etc. The hydrocarbon monomers are preferred for grafting purposes.

Also there can be used monomers such as alkyl arcylates and methacrylates, e.g., methyl acrylate, ethyl acrylate, methyl methacrylate, butyl acrylate, octyl acrylate, butyl methacrylate, octyl rnethacrylate, N,N-methylenebis-acrylamide, polyallyl esters, e.g., diallyl phthalate, diallyl oxalate, ethylene glycol dimethacrylate, triallyl cyanurate, bis-allyl carbonates, dially maleate, and diallyl fuma-rate, dialkenyl oxalates, e.g. diallyl oxalate, triallyl melamine, dialkyl maleates and fumarates, e.g., diethyl maleate and diethyl fumarate, etc.

Generally, from 5 to 30% of monomer is employed in forming the graft polymer. While the monomer may be grafted onto polyethylene in conventional fashion before irradiation, preferably the polyethylene and monomer are reacted together during the novel irradiation treatment.

While irradiation can be carried out at room temperature, or even below, it is preferably done at or slightly above the transition point of the polyethylene. Accordingly, the monomers used should not boil substantially below such temperature. Monomers, such as isoprene, can be dissolved in fluid polyethylene or other solvent. Similarly, the irradiation can be carried out while the polyethylene and other polymers or monomers are emulsified in water.

While it is not essential to use an additional catalyst, it is frequently desirable to add a free radical engendering material to hasten the cross-linking and/or polymerization. Conventional materials can be employed, such as peroxy compounds, e.g., benzoyl peroxide, p-chlorobenzoyl peroxide, methyl ethyl ketone peroxide, di-t-butyl peroxide, cumene hydroperoxide, acetyl peroxide, hydrogen peroxide, persulfates, e.g., potassium, ammonium and sodium persulfate, percarbonates, e.g., sodium percarbonate, perborates, e.g., sodium perborate, azo compounds of the type described in Hunt, Patent No. 2,471,959, e.g., azo-bis-isobutyronitrile, any of the peroxides recited in Schildknecht, Vinyl and Related Polymers (1952), page 78, dicumyl peroxide, etc.

The polyethylene employed should be one which is solid at room temperature and may have a molecular weight of 7,000; 12,000; 19,000; 21,000; 24,000; 30,000; 35,000, or even higher. For best results, the molecular weight should be at least 12,000 and, preferably, the molecular weight is about 20,000 to 28,000. The transition point of the polyethylene is generally about 105 to 125 C.

The following examples illustrate typical methods of carrying out the invention. Unless otherwise stated, all parts are parts by weight.

Example I A mixture of 90 parts of polyethylene, molecular weight 20,000, and 10 parts of solid polyisobutylene of molecular weight 168,000, were blended together on a rubber mill into a sheet 10 mils thick. Strips one inch long by /2 inch wide were cut from the sheet.

A Van de Graff electrostatic generator, manufactured by the High-Voltage Engineering Corporation, Cambridge, Mass., was used as a source of a beam of electrons. This machine is a high voltage X-ray generator and was modified by removal of the tungsten target from the path of the electrons to thereby permit irradiation of objects at the port. The generator was operated at two million volts with an amperage of 41 microamps at the target area per inch of scan.

A strip heater, equipped with a Variac, was fitted with a clamp to hold a flat Vycor glasscrucible cover. The crucible cover was the container for the polyethylenepolyisobutylene strip. The Variac was adjusted to give a temperature of 120 to 130 C. (previously measured by a thermocouple), and held at this temperature. The strips of the polymer blend were heated in the crucibles to 120 to 130 C. (slightly above the transition point). This required about 30 seconds each. The dishes were then inserted at the port of the machine, centered and irradiated. Exposure periods for the irradiation of the polyethylene-polyisobutylene were (a) 2 seconds (b) 10 seconds 30 seconds, and (d) 60 seconds Some improvement in the toughness and rigidity of the polymer blend was noted in irradiated sample (a) and these properties were further improved as the time of treatment was increased up to 60 seconds (sample d), which sample still had some flexibility.

Example II Example I(b) was repeated, replacing the polyisobutylene by 10 parts of a solid isobutylene-isoprene copolymer, containing 97% isobutylene units to form the sheet. The polymer blend was subjected to the electron beam recited in Example I until it had received a dosage of 20 l0 REP. The resulting strip was tougher and more rigid than the original polymer blend. Further strengthening can be obtained in this example, if parts of benzoyl peroxide are milled into the polymer blend prior to the irradiation. Alternatively, the peroxide could be added to the blend just prior to irradiation.

4 Example III parts of polyethylene, having a molecular weight of about 20,000 (Alathon 14) and 20 parts of soft, viscous polyisobutylene (Vistanex LM, Type MH) were blended and then irradiated in the form of a disk having a thickness of 40 mils at room temperature with the Van de Graif 2 m.e.v. electron accelerator. The dosage was 20x10 REP. The resulting product was still flexible but much more tear-resistant than a corresponding blend which had not been irradiated. The process of Example III was also carried out with a ratio of 30 parts polyethylene to 70 parts polyisobutylene or with a ratio of 70 parts polyethylene to 30 parts polyisobutylene.

Example IV parts of polyethylene (Alathon 14), 10 parts of distilled styrene (monomer) and one part of benzoyl peroxide were irradiated at room temperature, as in Example III, until the dosage was 20 10 REP. The irradiated graft polymer was considerably stronger than a similar graft polymer formed without the aid of the irradiation. Methyl ethyl ketone peroxide was used with equal effect in place of benzoyl peroxide in Example IV.

Example V 100 parts of polyethylene of molecular weight 20,000 (PM-1), 5 parts of para-divinyl benzene and 5 parts of benzoyl peroxide were irradiated at room temperature with the same electron beam as in Example III, until the dosage was 20x10 REP. The resulting graft polymer was translucent, flexible and very tough. In the absence of the irradiation, a less tough graft polymer was obtained which also had objectionable yellowish-brown staining (entirely absent from the irradiated product).

Example VI 60 parts of polyethylene (Alathon 14) and 40 parts of low viscosity depolymerized natural rubber, molecular weight 5,000, were blended and irradiated as in Example V, with a dosage of 20x10 REP. The cross-linked product was noticeably tougher than the non-irradiated blend. The irradiated polymer blend, furthermore, did not exude the depolymerized rubber while the non-irradiated blend did exude this material to an objectionable extent and also was quite sticky.

In place of the low viscosity depolymerized rubber in this example, there can be employed high viscosity depolymerized natural rubber, molecular weight about 8,000.

Example VII Example IV was repeated, omitting the peroxide. The irradiated graft polymer was tough and flexible, although not as tough as the irradiated product of Example IV.

Example VIII Example III was repeated, replacing the 20 parts of polyisobutylene with 20 parts of polymeric butadiene 1,3 to obtain a cross-linked polymer. Similarly, polychloroprene was used in place of the polyisobutylene in Example III -to obtain a cross-linked polymer.

While the results obtained, when polyethylene mixed with a polymer of a conjugated diolefin or halodiolefin, are good, it is again emphasized that the most outstanding advantage of the present invention is in the ability to improve the properties of hydrocarbon materials, such as polyisobutylene and Butyl rubber, which, by themselves, are degraded by irradiation. It should be noted that such materials are not the equivalents of the highly unsaturated polymers, such as GR-S and natural rubber.

It has generally been found that a dosage level of 2X10 REP is necessary to obtain a significant increase in strength and toughness of the product. At 6 l0 REP, the improvement is pronounced. In ascending order of dosage, these properties are progressively enhanced, being quite good at 20 10 REP. Increasing rigidity is also noted, being quite pronounced by 20x10 REP. In fact, above 50X 10 REP, it becomes difficult to mold the products by conventional molding techniques and it is necessary to resort to pressure and vacuum post forming at dosages from 50 to 100x10 REP. At dosages of 200x10 REP and above, a permanent amber tint is imparted to products made from a substantial proportion of polyethylene.

A REP, as is recognized in the art, is defined as that amount of nuclear radiation which dissipates 93 ergs of energy per gram of tissue producing l.61 10 ion pairs in the process. It is approximately equal to the amount of energy that would be dissipated by a one roentgen X- ray beam in a gram of tissue.

As the amount of irradiation dosage administered is increased, the polyethylene-polymer, e.g., polyisobutylene or monomer, e.g., styrene, blend diminishes in thermoplas ticity until, finally, transformation is effected into a thermosetting material.

The irradiated polyethylene-polymer monomer blends of the present invention can be formed into particles in the customary manner, for example, by chopping into pellets and then the pellets can be softened and formed into films or other shapes. As an example of a molded object, the polyethylene-polyisobutylene irradiated polymer of Example I(c) can be heated and formed into a cup. Alternatively, a disk of the irradiated polymer of Example III could be placed in a crown cap and molded into a cap liner with the aid of a two-step plunger.

The polyethylene-polymer or monomer blend can be irradiated at a thickness of from 3 mils or less (e.g., 1 mil), up to 100 mils or even higher, e.g., 250 mils, but is preferably about 40 to 50 mils. For thicknesses above 250 mils, the voltage should be above 2,000,000 and can be 3,000,000 or 6,000,000, or even higher. The lower limit of the voltage also is not particularly critical and can be 750,000 or below. In any event, the voltage should be sufiiciently high to induce the cross-linking necessary to give the desired increase in strength.

Ozone has an adverse effect upon polyethylene and the graft polymers thereof. Consequently, it is frequently desirable to have good ventilation or to carry out the irradiation while the polyethylene-polymer or monomer blend is in an atmosphere of inert gas, such as nitrogen or argon. Thus, the irradiation process of Example III can be carried out while continuously passing a stream of argon over the polyethylene-polyisobutylene blend.

It is also sometimes desirable to carry out the irradiation while the blend is maintained in a vacuum, e.g., 1 mm. or less. Thus, the irradiation in Example III can be carried out while the polyethylene-polyisobutylene blend is in a vacuum of 0.1 mm. total pressure.

While irradiation is preferably carried out with electrons, as set forth above, it is also possible to use other means of irradiation. Thus, if the tungsten target is put back, the machine described in Example I will permit X- rays to hit the polyethylene by placing the polymer by the side of the target. However, irradiation with X-rays takes longer than with electrons to obtain the same effect. It is also possible to irradiate with ,B-rays, e.g., by employing cobalt 60, carbon 14, phosphorus 32, or strontium 90, as a source of irradiation. Gamma-rays can be used, e.g., by submitting the polyethylene to irradiation from iron 59 or cobalt 60. Neutrons, protons, a-particles and deuterons also may be employed to bombard the polyethylene.

Instead of using the Van de Graft electrostatic generator as the source of the electron beam, other sources of high energy electrons can be employed, such as the General Electric 800,000 volt resonant transformer unit described by Lawton et al. in Industrial and Engineering Chemistry, volume 46, pages 1703 to 1709.

There can also be employed other conventional apparatus for producing beams of electrons, such as those recited, for example, in Brophy, Patent No. 2,668,133, column 3, lines 5 to 29.

As previously pointed out, for many useful results, the irradiation dose should be about 20 10 REP and the polyethylene should have a molecular weight before irradiation of about 20,000 or above. With polyethylene having a molecular weight of 7,000, it is necessary to employ a dosage of at least 100 10 REP in order to get satisfactory cross-linking and even higher dosages are necessary for lower molecular weight polymers. Correspondingly lower dosages can be employed to obtain cross-linking of higher molecular weight polymers.

The polyethylene-polyisobutylene, e.g., parts polyethylene mol. wt. 28,000 and 20 parts solid Vistanex, (or other polymer blend) can be shaped, e.g., in the form of a bottle and then irradiated to increase the strength of the formed article.

The cross-linked irradiated products of the present invention also have increased heat resistance, in contrast to the non-irradiated blends. In addition to the uses previously mentioned, the products of the invention can be employed in making flexible bags, squeeze bottles, beakers, etc.

We claim:

1. A process comprising irradiating normally solid polyethylene at a dosage of at least about 2x10 REP admixed with polyisobutylene to form a cross-linked copolymer.

2. A process according to claim 1 wherein the irradiation is by means of electrons.

3. A process according to claim 2 wherein the polyethylene is present in amount of about 5-95% of the irradiated product and the radiation with the electrons is carried out at a voltage of at least about 750,000 electron volts.

4. A process comprising irradiating normally solid polyethylene at a dosage of between about 2 10 REP and 200x10 REP with high energy radiation equivalent to at least about 750,000 electron volts admixed with polyisobutylene to form a cross-linked polyethylene co- References Cited in the file of this patent FOREIGN PATENTS Switzerland July 16, 1952 Belgium Nov. 28, 1952 OTHER REFERENCES Lawton et al., Nature, July 11, 1953, pages 76, 77. 

1. A PROCESS COMPRISING IRRADIATING NORMALLY SOLID POLYETHYLENE AT A DOSAGE OF AT LEAST ABOUT 4X10**6 REP ADMIXED WITH POLYISOBUTYLENE TO FORM A CROSS-LINKED COPOLYMER. 